Fuel controls for gas turbine engines operate in a closed loop fashion based upon a selected parameter, herein referred to as the primary parameter. The parameter may be a particular engine pressure ratio, compressor speed, or temperature at a selected location within the engine, such as the turbine exhaust temperature, or any other suitable parameter. Typically, a desired value of the parameter is continuously recomputed based upon throttle setting, empirically developed schedules, and other relevant information. This value is continuously compared to the actual value of the parameter; and fuel is modulated to drive the difference (i.e. error) to zero.
More specifically, when the pilot moves the throttle, the new throttle position, along with other information, is input into a control system which includes a schedule of curves for selecting an appropriate new value for the engine control parameter. The difference between the current actual value of that parameter and the value according to the control schedule is continuously calculated to produce an error term. The error term is compensated for the dynamic characteristics of the engine, and is converted to a fuel flow rate error term which, when used to control the engine, tells the engine the rate at which the fuel flow needs to be changed in order to drive the compensated error term to zero. Compensating the error term accounts for the time it takes the engine to respond physically to a change in fuel flow. Compensators are well known in the art.
While the engine is being operated based upon a primary parameter, the other parameters "float" in dependent fashion. It is desired to prevent these dependent parameters from exceeding certain Government agency certified limits. A control "topping loop" is used to accomplish this.
For example, in a twin spool gas turbine engine, there may be a Government certified steady state high pressure compressor speed (N.sub.2) which should not be exceeded. This maximum speed is referred to as the N.sub.2 steady state redline limit. In a prior art topping loop the difference between the steady state redline limit and the actual compressor speed is continuously calculated; and that difference or error term is passed through a compensator of the type discussed above and is converted to a compensated fuel flowrate error term. As also discussed above, a fuel flowrate error term is being continuously calculated in the primary control loop based upon primary parameter schedules. Both flowrate error terms are delivered to a select-low gate. The lowest of these flowrate error terms is thereupon used by the control to adjust the engine fuel flow.
When the engine is operating well below the N.sub.2 redline limit, the topping loop flowrate error term will be very large compared to the primary parameter flowrate error term. The primary parameter error term will be selected under those circumstances. If the value of the high pressure compressor speed (N.sub.2) gets close enough to the steady state redline limit, the topping loop flowrate error term will fall below that of the primary parameter flowrate error term and will be selected to control the engine fuel flow. The object is to prevent the engine control from selecting a rate of fuel flow which could cause the compressor speed to exceed the redline limit; however, in some engines this has not been possible to achieve for the entire engine operating envelope. The reason for occasional failure is that the compensator must be designed to always allow the engine to accelerate at specification minimum rates; and the control must also permit the engine to operate, at steady state, up to the redline limit. Under certain conditions, and using prior art topping loop techniques, these requirements are incompatible with avoiding exceeding the redline limit. Therefore, prior art topping loop compensators for the high pressure compressor speed error term have been designed to allow transient overshooting of the redline limits under certain circumstances. While this overshooting is brief and does not pose a safety problem, it does raise an automatic flag in the cockpit, requiring service personnel to determine whether there has been an engine malfunction. The pilot will also see this flag, and may believe there is a problem when there actually is none.
Users of engines with the prior art control system topping loops desire to eliminate this redline overshooting situation in a manner which allows the engine to meet acceleration specifications and to operate up to the redline limit during steady state operation over the entire flight envelope.